Liver cancer (or hepatic cancer) is a cancer that originates in the liver. Primary liver cancer is the fifth most frequently diagnosed cancer globally and the second leading cause of cancer death.
Liver cancers are malignant tumours that grow on the surface or inside the liver. They are formed from either the liver itself or from structures within the liver, including blood vessels or the bile duct.
The leading cause of liver cancer is viral infection with hepatitis B virus or hepatitis C virus. The cancer usually forms secondary to cirrhosis caused by these viruses. For this reason, the highest rates of liver cancer occur where these viruses are endemic, including East-Asia and sub-Saharan Africa. Liver cancers should not be confused with liver metastases, also known as secondary liver cancer, which is a cancer that originate from organs elsewhere in the body and migrate to the liver.
The most frequent liver cancer, accounting for approximately 75% of all primary liver cancers, is hepatocellular carcinoma (HCC). HCC is a cancer formed by liver cells, known as hepatocytes that become malignant. Another type of cancer formed by liver cells is hepatoblastoma, which is specifically formed by immature liver cells. It is a rare malignant tumour that primarily develops in children, and accounts for approximately 1% of all cancers in children and 79% of all primary liver cancers under the age of 15.
Liver cancer can also form from other structures within the liver such as the bile duct, blood vessels and immune cells. Cancer of the bile duct (cholangiocarcinoma and cholangiocellular cystadenocarcinoma) accounts for approximately 6% of primary liver cancers. There is also a variant type of HCC that consists of both HCC and cholangiocarcinoma. Tumours of the liver blood vessels include angiosarcoma and hemangioendothelioma. Embryonal sarcoma and fibrosarcoma are produced from a type of connective tissue known as mesenchyme. Cancers produced from muscle in the liver are leiomyosarcoma and rhabdomyosarcoma. Other less common liver cancers include carcinosarcomas, teratomas, yolk sac tumours, carcinoid tumours and lymphomas. Lymphomas usually have diffuse infiltration to liver, but it may also form a liver mass in rare occasions.
Surgical resection is often the treatment of choice for non-cirrhotic livers. Increased risk of complications such as liver failure can occur with resection of cirrhotic livers. 5-year survival rates after resection has massively improved over the last few decades and can now exceed 50%.
Recurrence rates after resection due to the spread of the initial tumour or formation of new tumours exceeds 70%. Liver transplantation can also be used in cases of HCC where this form of treatment can be tolerated and the tumour fits specific criteria (e.g., the Milan criteria). Less than 30-40% of individuals with HCC are eligible for surgery and transplant because the cancer is often detected late stage. Also, HCC can progress during the waiting time for liver transplants, which can ultimately prevent a transplant.
Percutaneous ablation is the only non-surgical treatment that can offer cure. There are many forms of percutaneous ablation, which consist of either injecting chemicals into the liver (ethanol or acetic acid) or producing extremes of temperature using radio frequency ablation, microwaves, lasers or cryotherapy. Of these, radio frequency ablation has one of the best reputations in HCC, but the limitations include inability to treat tumours close to other organs and blood vessels due to heat generation and the heat sync effect, respectively.
Systemic chemotherapeutics are not routinely used in HCC, although local chemotherapy may be used in a procedure known as transarterial chemoembolization (TACE). In this procedure, cytotoxic drugs such as doxorubicin or cisplatin with lipiodol are administered and the arteries supplying the liver are blocked by gelatine sponge or other particles. Because most systemic drugs have no efficacy in the treatment of HCC, research into the molecular pathways involved in the production of liver cancer produced sorafenib, a targeted therapy drug that prevents cell proliferation and blood cell growth in some circumstances. In further research, the fluoro analogue of sorafenib, regorafenib was produced. Regorafenib is a targeted therapy drug that is an oral receptor tyrosine kinase inhibitor which blocks an important pathway that promotes cell division.
Radiotherapy is not often used in HCC because the liver is not tolerant to radiation. Even with modern technology providing well targeted radiation to specific areas of the liver, collateral damage to surrounding liver tissue is a problem, emphasizing the need for better, “liver sparing” regimens. Dual treatments of radiotherapy plus chemoembolization, local chemotherapy, systemic chemotherapy or targeted therapy drugs may show benefit over radiotherapy alone.
Sorafenib (marketed as NEXAVAR®), is an FDA-approved drug for patients with advanced primary liver cancer. It is a small molecule interacting with multiple intracellular and cell surface kinases including the Raf/Mek/Erk pathway. By inhibiting these kinases, genetic transcription involving cell proliferation and angiogenesis is inhibited, with the intriguing observation that hypoxia in solid tumour tissues may be increased due to the treatment reducing blood supply to the tumour. However, even with the development of drugs like sorafenib, the current treatment options for liver cancer are insufficient due to its limited effectiveness and severe toxicity.
Regorafenib (marketed as STIVARGA®), is an FDA-approved drug for metastatic colorectal cancer patients who failed with standard treatments, and for patients with advanced gastrointestinal stromal tumors (GIST) that cannot be surgically removed and no longer respond to other FDA-approved treatments for this disease. Regorafenib has also shown positive results in terms of time to progress (TTP) and overall survival (OS) in a phase II clinical trial as a second-line drug for patients with liver cancer who progress after sorafenib treatment.
Troxacitabine, (beta-L-dioxolane cytidine) is a cytotoxic deoxycytidine analogue with an unnatural L-configuration which has demonstrated broad activity against both solid and hematopoietic malignancies in vitro and in vivo. Particularly, impressive activity has been observed against human cancer cell lines and xenografts of hepatocellular, prostate, and renal origin (Cancer Res., 55, 3008-3011, 1995). Troxacitabine treatment has shown to give rise to a resistance mutation of the kinase deoxycytidine kinase (dCK) which is normally responsible for the first phosphorylation step of the nucleoside, leading to no or very low levels of troxacitabine monophosphate.
Troxacitabine entered phase III clinical trials in 2008 in the acute myologenous leukemia indication, but did not proceed to registration. Discontinued phase II trials with troxacitabine include breast cancer, colorectal cancer, pancreatic cancer, melanoma, NSCLC, renal, prostate and ovarian tumours. Troxacitabine was generally administered as an intravenous infusion, thereby exposing many tissues to the drug, irrespective of the site of the cancer. It is believed that the clinical development of troxacitabine has been abandoned.